1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating a semiconductor device. For example, the present invention relates to a crystal structure of a solder bump to be a connection terminal of a flip chip semiconductor device or a method for forming a solder bump.
2. Related Art
Nowadays, a flip chip model, in which a solder bump to be a connection terminal of a semiconductor substrate on which a semiconductor device is formed and a solder bump to be a connection terminal of a package-side circuit substrate on which the semiconductor substrate is to be packaged are linked, is adopted. Particularly in a solder bump formation process of a semiconductor substrate on which a semiconductor device is formed, a layer called UBM (Under Bump Metal) is formed and a resist pattern patterned by using a photo resist is formed on the UBM layer. Then, the opened UBM layer is plated with a material to be a solder bump, and the photo resist and the UBM layers thereunder are removed, thereby forming plating to be a bump material (see Published Unexamined Japanese Patent Application, Publication No. 2004-207685 (JP 2004-207685A), for example). In JP 2004-207685A, a method for fabricating a tin-copper (SnCu) laminated bump, in which a photo resist mask is formed on UBM and then the photo resist mask is plated with copper (Cu) and subsequently with tin (Sn), is described for example. Nowadays, due to environmental problems, lead-free solder containing no lead (Pb) is used even for solder.
However, if a solder bump is formed simply by the method of JP 2004-207685A, there is a problem that bump cracks appear due to a contraction or the like caused by thermal change. Particularly, in many cases, stress concentration occurs in an outer circumferential part (end part) of a connection part of a bump and a semiconductor substrate and from this point, a crack grows into bumps. Thus, if a crack propagates into a bump, the solder bump to be a connection terminal may be broken or, if not broken, resistance thereof will increase. Such problems significantly decrease reliability of solder bumps. However, neither effective method as countermeasures against problems such as bump cracks nor product for which such measures have been taken has been presented.